Technology update

Nov 4, 2011

Silicon carbide shows promise for quantum computing

Silicon carbide, a material that is already widely used in high-power electronics, may also be a candidate for quantum information processing. So say researchers at the University of California at Santa Barbara who have studied point defects in the material. These defects, similar to ones found in diamond, possess electron spin states that can be coherently controlled and manipulated as quantum bits (qubits) using light.

While classical computers store and process information as "bits" that can have one of two logic states – "0" or "1" – a quantum computer exploits the ability of quantum particles to be in "superposition" of two or more states at the same time. Such a device could, in principle, outperform a classical computer on certain tasks, like code decryption for example, because its processing speed should increase exponentially with the number of qubits of information involved. In practice, however, physicists have struggled to create even the simplest quantum computer because the fragile nature of these quantum states means that they are easily destroyed and are difficult to control.

Recently, diamond-based qubits created a flurry of interest because their "decoherence" time (the time that they can retain their logic state) is much longer than the time it takes to performs qubit operations – even at room temperature. What is more, they can be read out optically, which means that they could potentially be integrated into photonic quantum-information processing systems. However, making these qubits remains a challenge – both in terms of scaling up and cost.

Now, in a new paper in Nature, David Awschalom and colleagues are saying that there exist defect qubits in silicon carbide that are very similar to those of diamond. Silicon carbide is already widely used in high-power electronics, thanks to its high thermal conductivity and high maximum current density to name but two good properties, and so could more easily be scaled up to larger systems than diamond can.

Divacancies
The researchers looked at a polytype (a crystal structure) of silicon carbide called 4H-SiC that contains naturally occurring defects (or "divacancies"). These defects, which correspond to a missing silicon atom next to a missing carbon atom in the crystal, are very much like the defects in diamond known as "nitrogen-vacancy centres" – that form when a nitrogen impurity finds itself next to a missing carbon atom in the diamond lattice. But, the good thing is that both types of defect form a muti-electron system that has a net angular momentum (or spin) that can be aligned either parallel ("1") or antiparallel ("0") to an applied magnetic field, and can so be exploited as a qubit. Some SiC divacancies can also be accessed optically and have long decoherence times at room temperature – just like those in diamond.

Like previous experiments on diamond nitrogen vacancy centres, Awschalom and co-workers measured the spin of the divacancies in 4H-SiC using photoluminescence. This involved shining laser light onto the sample and collecting the fluorescence light subsequently emitted by it. And, as for diamond nitrogen vacancies, the fluorescence of the silicon carbide divacancies depends on their spin state, so it is thus possible to "read out" the state of the qubits in this way.

Quantum writing
By then applying an oscillating magnetic field at microwave frequencies to the sample, the researchers were able to perform electron spin resonance. Here, the spin of a divacancy oscillated between its two-qubit states, something that can be used to quantum "write" to the sample. Again, the technique has already been tried and tested in diamond.

It all seems to be too good to be true, but Andrew Dzurak of the University of NSW cautions in a related News & Views article: "Although silicon carbide qubits offer enticing prospects for quantum computing, a number of challenges for this new technology remain. First, the qubit operations reported here were performed on a large ensemble of qubits, so the next step will be to demonstrate control and measurement of a single qubit. More significantly, technologies must be developed to 'engineer' thousands of individually addressable divacancy qubits, rather than merely identifying accidentally located defects. Engineering will also be needed to configure pairs of adjacent qubits reliably, to enable controlled two-qubit operations – another vital requirement for quantum computation."

However, if these challenges can be met, silicon carbide could become a "serious candidate" for large-scale quantum computing, he says.